An induction heating (IH) type of fixing apparatus generates an eddy current through the action of a magnetic field generated by a magnetic field generation section in a heat-producing element, and heat-fixes an unfixed image on a recording medium such as transfer paper or an OHP sheet through Joule heating of the heat-producing element by means of the eddy current.
An advantage of this induction heating type of fixing apparatus compared with a heat roller type of fixing apparatus that uses a halogen lamp as a heat source is that heat production efficiency is higher and the fixing speed can be increased.
With a fixing apparatus that uses a thin heat-producing element comprising a thin sleeve or endless belt as the heat-producing element, the thermal capacity of the heat-producing element is low and the heat-producing element can be made to produce heat in a short time, enabling startup responsiveness until heat production at a predetermined fixing temperature to be markedly improved.
On the other hand, with a fixing apparatus that uses this kind of heat-producing element of low thermal capacity, heat is lost simply through the passage of a recording medium, causing a drop in the temperature of the paper passage area. Therefore, with this kind of fixing apparatus, the heat-producing element is heated in a timely fashion so that the temperature of the paper passage area is maintained at a predetermined fixing temperature.
Consequently, with a fixing apparatus that uses this kind of heat-producing element of low thermal capacity, if recording media of small size are fed through continuously, the heat-producing element is continuously heated, and a phenomenon whereby the temperature of a paper non-passage area becomes abnormally higher than the temperature of a paper passage area—that is, a phenomenon of an excessive rise in temperature of a paper non-passage area—occurs.
A known technology for eliminating this kind of phenomenon of an excessive rise in temperature of a paper non-passage area is one whereby, of the magnetic flux generated by a magnetic flux generation section that performs induction heating of the heat-producing element, only magnetic flux that acts on a paper non-passage area of the heat-producing element is absorbed by a magnetic flux absorption member capable of moving in the heat production width direction of the heat-producing element (see, for example, Patent Document 1).
Another known technology for eliminating the phenomenon of an excessive rise in temperature of a paper non-passage area is one whereby a second core of magnetic material corresponding to a paper non-passage area is positioned at the rear of a first core of magnetic material of a magnetic flux generation section that causes heat generation of a heat-producing element by electromagnetic induction, and the lengthwise temperature distribution of the heat-producing element is changed by varying the gap between the first core of magnetic material and second core of magnetic material (see, for example, Patent Document 2).
FIG. 1 is a schematic oblique drawing of a sample implementation of a fixing apparatus disclosed in Patent Document 1. As shown in FIG. 1, this fixing apparatus is provided with a coil assembly 10, a metal sleeve 11, a holder 12, a pressure roller 13, a magnetic flux masking shield 31, a displacement section 40, and so forth.
In FIG. 1, coil assembly 10 generates a high-frequency magnetic field. Metal sleeve 11 is heated by an induction current induced by an induction coil 18 of coil assembly 10, and rotates in the direction of transportation of recording material 14. Coil assembly 10 is supported inside holder 12. Holder 12 is fixed to a fixing unit frame (not shown) and does not rotate. Pressure roller 13 rotates in the direction of transportation of recording material 14 while pressing against metal sleeve 11 and forming a nip area. By having recording material 14 gripped and transported by means of this nip area, an unfixed image on recording material 14 is heat-fixed to recording material 14 by metal sleeve
As shown in FIG. 1, magnetic flux masking shield 31 exhibits an arc-shaped curved surface that mainly covers the upper half of induction coil 18, and is advanced and withdrawn with respect to the gap between both ends of coil assembly 10 and holder 12 by means of displacement section 40. Displacement section 40 has a wire 33 linked to magnetic flux masking shield 31, a pair of pulleys 36 on which wire 33 is suspended, and a motor 34 that rotates one of the pulleys 36.
When the size of recording material 14 is the maximum size, magnetic flux masking shield 31 is moved by means of displacement section 40 so as to be withdrawn into the position shown by the solid line in FIG. 1. On the other hand, when the size of recording material 14 is small, magnetic flux masking shield 31 is moved so as to advance into the position shown by the dot-dot-dash line in FIG. 1. By this means, magnetic flux reaching a paper non-passage area of metal sleeve 11 from induction coil 18 is masked, and an excessive rise in temperature of a paper non-passage area is suppressed.
FIG. 2 shows schematic cross-sectional views of a sample implementation of a fixing apparatus disclosed in Patent Document 2. As shown in FIG. 2, this fixing apparatus is provided with a heating assembly 51, a holder 52, a core-holding rotating member 53, an exciting coil 54, a first core 55, a second core 56, a fixing roller 57, a pressure roller 58, and so forth.
In FIG. 2, heating assembly 51 is composed of holder 52, core-holding rotating member 53, exciting coil 54, first core 55, and second core 56, and generates magnetic flux. Fixing roller 57 is induction-heated through the action of magnetic flux generated by heating assembly 51, and rotates in the direction of transportation of recording material 59.
Pressure roller 58 rotates in the direction of transportation of recording material 59 while pressing against fixing roller 57 and forming a nip area. By having recording material 59 gripped and transported by means of this nip area, an unfixed image on recording material 59 is heat-fixed to recording material 59 by heated fixing roller 57.
First core 55 has the same width as the width of the maximum paper passage area of fixing roller 57. When the size of recording material 59 is the maximum size, second core 56 is moved to a position close to first core 55, as shown in FIG. 2A. On the other hand, when the size of recording material 59 is small, core-holding rotating member 53 rotates through 180° and second core 56 is moved to a position away from first core 55, as shown in FIG. 2B. By this means, heat production of a paper non-passage area of fixing roller 57 corresponding to second core 56 is suppressed.    Patent Document 1: Unexamined Japanese Patent Publication No. HEI10-74009    Patent Document 2: Unexamined Japanese Patent Publication No. 2003-123961